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Prem Krishnan Raghupathi:
Impact of free living protozoa and bacterial interactions on multispecies biofilm

Date: 15-07-2018    Supervisor: Søren J. Sørensen & Mette Burmølle

Interactions between bacteria belonging to different species are vital for the development of complex microbial communities, including multispecies biofilm. Multispecies biofilms are ubiquitous in most natural and man-made environments; their presence is now subject to ever-increasing attention. Several studies have shown that bacterial species living in complex bacterial communities interact, both intra- and interpecifically, and that these interactions are instrumental in structural establishment and distribution of bacterial species within multispecies biofilm. These complex interactions often result in the bacteria developing properties that would not been present when grown alone. These emergent properties include increased tolerance to antibiotics, host immune responses, and other stressors, which has proven to provide increased fitness benefits to members of the mixed community. Co-cultivation studies using in-vitro multispecies settings have shown that bacteria in mixed communities produce increased biomass, and many studies have documented the formation of microbial aggregates, microcolonies or biofilm formation in response to the presence of predatory protozoa. The threat from bactericidal protozoans can affect the physiological state of the bacterial community and result in bacterial responses at both species and social levels, which is in turn influenced by the combination of different interactions and parameters.

The purpose of this Ph.D. thesis was to address various aspects of bacterial interactions, all of which support multispecies biofilm formation, and to investigate the role of biofilms as protective mechanisms when grazing is widespread. More specifically, the microbial diversity of multispecies biofilm and selected eukaryotic organisms (protozoa and fungi, reespectively) associated with toothbrushes (manuscript 1) and dishwashers (manuscripts 2 and 3) were investigated. Multiple bacterial communities isolated from dishwashers were screened for their ability to produce biofilm; both individually and in co-cultures. The influence of bacterial interactions on population dynamics in a model culture with four different bacterial strains exposed to grazing (manuscript 4) was also studied.

The results presented in this thesis shows that studies conducted under the conditions of multiple species, even though they are less complex than naturally occurring bacterial communities, allow us to characterize biofilms representing their natural environments where they most often exist as multispecies microbial communities. The resulting emerging properties such as increased biomass production and fitness benefits (protection against grazing) associated within the biofilm architecture, substantiate the presence of synergistic interactions in multispecies biofilm and further emphasize their influence on individual bacterial species during biofilm formation.